U.S. Pat. Nos. 5,665,554 and 5,523,231 describe the use of magnetic beads to isolate total DNA via precipitation. The DNA is precipitated in the presence of magnetic beads that do not specifically associate with the beads. The beads become entrapped within the precipitate and can be used to remove the precipitate without centrifuging.
A different method for purification and isolation of DNA using magnetic beads has been described U.S. Pat. No. 5,705,628. In this method, carboxyl coated magnetic micro particles (1 μm in diameter) are being used to bind DNA non-specifically in the presence of varying concentrations of salt and PEG. The method may selectively bind to nucleic acid of different sizes by adjusting the salt and PEG concentrations.
Yet another method for extracting nucleic acid from biological material is described in EP 1036082. A commercial product used in this method for PCR clean-up is the ChargeSwitch® kit from Invitrogen. According to the manufacturer, this kit includes magnetic beads immobilized with the ChargeSwitch ligand. The ligand appears to have a positive charge at pH<6.5, which becomes neutral at pH>8.5. The protocol is in three steps:    1. The ChargeSwitch® ligand acquires a positive charge at low pH values (<6.5) and binds DNA and RNA through their negatively charged phosphate backbone.    2. Proteins and other impurities can be separated from the ChargeSwitch-bound nucleic acids through the use of aqueous wash buffers.    3. Nucleic acids can then be released from the ChargeSwitch® ligand when the pH of the surrounding media is raised (>8.5) and the positive charge is neutralized.